The relationship of membrane fluidity to growth and nuclear cycle in mitogen stimulated lymphocytes

Fluorescence polarization (P) and anisotropy (rs) of the diphenyl-hexatriene labeled, stimulated human lymphocytes were followed in relation to cell cycle progression. Fluorescence anisotropy is proportional to the structural order parameter of the apolar lipid region, thus, indicative of the membrane fluidity. Cell cycle progression was defined by monitoring the RNA, protein and DNA synthesis as well as the expression of interleukin 2 and transferrin receptors. Cellular growth in size was also measured. A decrease in P and rs values was seen when the RNA and protein synthesis as well as the density of growth factor receptors and cellular volume increased. At the same time no DNA synthesizing cell was detected. The low P and rs values remained unaltered throughout the cycle, thus proving to be independent from cell proliferation (DNA synthesis) and from further progressive increase in cellular volume. The data indicate that the structural change in the membranes is an early event of cell activation and occurs in the course of G0-G1 transition as part of the growth cycle.